c++: set TYPE_STRING_FLAG for char8_t

While looking at the DWARF handling of char8_t I wondered why we weren't
setting TREE_STRING_FLAG on it.  I hoped that setting that flag would be an
easy fix for PR102958, but it doesn't seem to be sufficicent.  But it still
seems correct.

I also tried setting the flag on char16_t and char32_t, but that broke
because braced_list_to_string assumes char-sized elements.  Since we don't
set the flag on wchar_t, I abandoned that idea.

gcc/c-family/ChangeLog:

	* c-common.cc (c_common_nodes_and_builtins): Set TREE_STRING_FLAG on
	char8_t.
	(braced_list_to_string): Check for char-sized elements.